Term: Introductory Sentences

Introductory sentences are general sentences that open paragraphs and precede the topic sentence.
They provide background about the topic or main idea.
Unlike topic sentences, introductory sentences are not developed throughout the paragraph

Introductory Sentences Versus Topic Sentences

Because textbook authors often make the first sentence of a paragraph the topic sentence,
readers sometimes confuse introductory sentences with topic sentences.
They assume, that is, that the first sentence in the paragraph expresses the main idea when, in fact, what it's doing is paving the way for the main idea, usually by providing some background. To avoid getting confused, readers need to know what makes an introductory sentence different from a topic sentence: Although both introductory and topic sentences are more general than the rest of the sentences in the paragraph, the introductory sentence is not further developed by the remaining sentences that follow it. In other words, you can't track references to it throughout the paragraph. The introductory sentence (or sentences) makes a point and isn't heard from again.

Comparing Introductory and Topic Sentences

Here to illustrate the difference between an introductory and a topic sentence are two paragraphs from a textbook.

Topic Sentence First: In the paragraph that follows, the topic sentence comes first and the remaining sentences pick up on the idea that babies are instinctively social:

Infants appear to have a natural tendency to be social. In responding to sounds, for instance, even newborns show a preference for the voices of their mothers. Immediately after birth, newborns will also turn their head toward the sound of a human voice and search for its source. Even in the midst of being fed, infants will pause at the sound of a human voice but continue sucking undisturbed at non-human sounds. Infants also seem to prefer the smell and taste of human milk over those of formula, water or sugar water. (Source of Information: Adapted from Kelvin L. Seifert and Robert J. Hoffnung. Child and Adolescent Development. Boston: Houghton Mifflin, 2000. p.200)

In this case, the first sentence is the topic sentence because it introduces the central idea of the paragraph:
Babies seem naturally inclined to be social. How do you know that sentence is the topic sentence expressing the main idea? You know it because every sentence that follows uses the word "human" to pick up on the claim that "babies seem naturally inclined to be social." In other words, they like to be around humans. Starting with sentence two, each of the following sentences illustrates babies instinctively responding to people more than objects, in other words being social creatures.

Introductory Sentence First: Now let's look at another paragraph, one where the topic sentence comes second rather than first. Note here how the second sentence does not continue the train of thought begun by the first. Instead the second sentence introduces a new idea. The remaining sentences then refer to, or pick up, on words from the second rather than the first sentence:

Deep sea divers know full well about the beauties that lie deep in the ocean's watery depths. But they also know that the sea has its dangers as well, among them the members of the family Dasyatidae, more commonly known as stingrays. Lying almost completely covered by sand on the ocean floor, stingrays respond almost immediately to the touch of a human hand or foot. In response to what they perceive as an attack, the stingrays usually whip their tails around and plant a sharp spine in the intruder's flesh. Because those spines have poison in them, wounds from a stingray can cause nausea, diarrhea, falling blood pressure, sometimes even death. Although there have been cases of stingrays becoming accustomed to and tolerating human beings who fed them, this is the exception, not the rule. Where stingrays are concerned, the rule is "Divers Beware."

The first sentence in the paragraph suggests the author is going to explore the beauty of life under water.
However, the second sentence challenges that expectation by discussing not the ocean's beauty, but its dangers.
More specifically, it focuses on the danger of encountering a sting ray.
At this point, experienced readers know two things:

Sentence 1 is an introductory sentence rather than a topic sentence, and

Sentence 2 might well be the topic sentence. What would confirm the expectations about the second sentence's role in the paragraph is the third sentence. If the third sentences continues to discuss the dangers of sting rays, then sentence 2 is definitely the topic sentence.

Since in the above paragraph, the third sentence continues the discussion of dangerous stingrays, it's clear that sentence 2 is not another introductory sentence but rather the topic sentence of the paragraph.

Responding to Introductory Sentences

Although topic sentences are often the first sentence in a paragraph, particularly in textbooks, they don't always appear in the opening position. Writers sometimes use the first, even the second or third, sentence to provide background. When this happens, you just need to keep looking for the sentence that includes words or phrases which are repeated or referred to throughout the entire paragraph. Introductory sentences won't have any such words or phrases. While a word or phrase in the introductory sentence might be repeated or referred to once, maybe even twice, the rest of the sentences in the paragraph will pursue a different train of thought than the one outlined in an introductory sentence. Keep that distinction in mind and you are unlikely to confuse an introductory comment with the main idea of a paragraph.